The present invention relates to a lagged pipe for transporting fluids and more particularly to a flexible pipe used for transporting hydrocarbons from a subsea well.
It is becoming increasingly necessary for flexible pipes to be well insulated because of the operating conditions under which offshore oil deposits are worked. This is because the heavy oils in these deposits tend to solidify while they are being transported between the subsea production well and the surface equipment, such as a platform, as a result of heat losses in the flexible pipe submerged in the sea. Good thermal insulation is also desirable in order to avoid the formation of hydrates to which certain crude oils are liable as they cool down.
The known prior solutions are described in FR-A-2,563,608. One of the improvements recommended by FR-A-2,563,608 consists in placing, around the internal core of the pipe, a plurality of annular partitions distributed along the length of the core and integral with the latter, in filling the annular chambers made between the successive partitions with an insulation material, in continuously extruding an outer sheath over the partitions, the space between two successive partitions being between 20 and 200 m depending on the service conditions. According to another improvement described in that document, the thermal insulation is obtained by spiralling hollow tubes around the central core, the tubes having a diameter of between 5 and 30 mm and a thickness of 0.5 to 4 mm. In all the embodiments, the bands of thermal insulation must offer a certain resistance to the hoop stress and a certain mechanical strength to be able to transfer the clamping loads to the tensile armour plies. Because of the short-pitch winding of these bands of thermal insulation, the corresponding layers tend to behave as rings and they therefore offer great resistance to radial deformation, such resistance being called the "vault effect" by the experts. When the number of thermal insulation layers is increased, as is the case in very deep applications, the force transferred to the tensile armour plies is relatively small compared with the force applied by the tensioners, as described below, which considerably reduces the capacity of the laying system. The vault effect becomes significant when nEe.sup.3 &gt;400 Nm, where n is the number of layers, E is the Young's modulus and e the thickness of one layer.
Pipes with such a thermal insulation exhibit what the experts call the "vault effect" and their applications are limited by the number of thermal insulation layers. It is important to note or recall that so-called rigid or flexible pipes, when they are being unwound from the holding reel or reels placed on a laying barge, pass through devices called tensioners whose purpose is especially to take up most of the weight of a pipe or of the tensile loads on the latter before it is submerged in the water. Such tensioner devices are well known and described, for example, in U.S. Pat. No 4,345,855. In order for these tensioners to fulfil their role fully, the maximum clamping force of the tensioners must be transferred, at least mostly, if not completely, to the central core of the pipe.
In a rigid or almost-rigid pipe, as in a pipe with an "external vault", the resistance to the transfer of the application force of the tensioners is too high and only part of the applied force is transferred to the central this is insufficient to take up the weight of the pipe or the tensile forces in the latter.
In a pipe thermally insulated by means of superposed layers of thin bands wound with a short pitch, that is to say with a wind angle of greater than 55.degree., or in a pipe of the type described in FR-A-2,563,608, the application force of the tensioners deforms the insulating structure very considerably at the clamping pads of the tensioners while causing the insulating structure between the pads to creep or expand, thus forming kinds of bulges between the pads. Curves of the force (F.sub.T) transferred to the central core as a function of the force (F.sub.P) applied by the pads of the tensioners, for various thicknesses e of a compliant insulating structure, show that as e increases, the transferred force (F.sub.T) becomes relatively low. However, if a thin insulating structure is produced, then the thermal insulation obtained is unsatisfactory or not effective enough.